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• iPhone: Which are the most useful techniques for faster Bluetooth?

  - by Mike Howard

  Hi. I'm adding peer-to-peer bluetooth using GameKit to an iPhone shoot-em-up, so speed is vital. I'm sending about 40 messages a second each way, most of them with the faster GKSendDataUnreliable, all serializing with NSCoding. In testing between a 3G and 3GS, this is slowing the 3G down a lot more than I'd like. I'm wondering where I should concentrate my efforts to speed it up. How much slower is GKSendDataReliable? For the few packets that have to get there, would it be faster to send a GKSendDataUnreliable and have the peer send an acknowledgement so I can send again if I don't get the Ack within, say, 100ms? How much faster would it be to create the NSData instance using a regular C array rather than archiving with the NSCoding protocol? Is this serialization process (for about a dozen floats) just as slow as you'd expect from an object creation/deallocation overhead, or is something particularly slow happening? I heard that (for example) sending four seperate sets of data is much, much slower, than sending one piece of data four times the size. Would I make a significant saving by sending separate packets of data that wouldn't always go together in the same packet when they happen at the same time? Are there any other bluetooth performance secrets I've missed? Thanks for your help.

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• C++ Virtual Methods for Class-Specific Attributes or External Structure

  - by acanaday

  I have a set of classes which are all derived from a common base class. I want to use these classes polymorphically. The interface defines a set of getter methods whose return values are constant across a given derived class, but vary from one derived class to another. e.g.: enum AVal { A_VAL_ONE, A_VAL_TWO, A_VAL_THREE }; enum BVal { B_VAL_ONE, B_VAL_TWO, B_VAL_THREE }; class Base { //... virtual AVal getAVal() const = 0; virtual BVal getBVal() const = 0; //... }; class One : public Base { //... AVal getAVal() const { return A_VAL_ONE }; BVal getBVal() const { return B_VAL_ONE }; //... }; class Two : public Base { //... AVal getAVal() const { return A_VAL_TWO }; BVal getBVal() const { return B_VAL_TWO }; //... }; etc. Is this a common way of doing things? If performance is an important consideration, would I be better off pulling the attributes out into an external structure, e.g.: struct Vals { AVal a_val; VBal b_val; }; storing a Vals* in each instance, and rewriting Base as follows? class Base { //... public: AVal getAVal() const { return _vals->a_val; }; BVal getBVal() const { return _vals->b_val; }; //... private: Vals* _vals; }; Is the extra dereference essentially the same as the vtable lookup? What is the established idiom for this type of situation? Are both of these solutions dumb? Any insights are greatly appreciated

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• xml parameter for soap webservice request in iphone

  - by Jayshree

  Hello everybody. I want to send a soap request to a webservice method. Now i want to send the parameter as an xml input for that request. so plz can anybody give me an idea of how to do it in iphone? Right now i am sending the request in following way : NSString *soapMessage = @"<?xml version=\"1.0\" encoding=\"utf-8\"?>\n" "<soap:Envelope xmlns:xsi=\"http://www.w3.org/2001/XMLSchema-instance\" xmlns:xsd=\"http://www.w3.org/2001/XMLSchema\" xmlns:soap=\"http://schemas.xmlsoap.org/soap/envelope\">\n" "<soap:Body>\n" "<GetCustomerInfoXML xmlns=\"http://qa2.alliancetek.com/phpwebservice\">\n" "<id><customer><id>1</id></customer></id>" "</GetCustomerInfoXML>" "</soap:Body>\n" "</soap:Envelope>"; NSLog(@"%@",soapMessage); NSURL *url = [NSURL URLWithString:@"http://qa2.alliancetek.com/phpwebservice/index.php"]; NSMutableURLRequest *theRequest = [NSMutableURLRequest requestWithURL:url]; NSString *msgLength = [NSString stringWithFormat:@"%d", [soapMessage length]]; [theRequest addValue: @"text/xml; charset=utf-8" forHTTPHeaderField:@"Content-Type"]; [theRequest addValue: @"http://qa2.alliancetek.com/phpwebservice/index.php//GetCustomerInfoXML" forHTTPHeaderField:@"SOAPAction"]; [theRequest addValue: msgLength forHTTPHeaderField:@"Content-Length"]; [theRequest setHTTPMethod:@"POST"]; [theRequest setHTTPBody: [soapMessage dataUsingEncoding:NSUTF8StringEncoding]]; But i want to send the parameters in xml format. so how do i do it????

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• C++ design question, container of instances and pointers

  - by Tom

  Hi all, Im wondering something. I have class Polygon, which composes a vector of Line (another class here) class Polygon { std::vector<Line> lines; public: const_iterator begin() const; const_iterator end() const; } On the other hand, I have a function, that calculates a vector of pointers to lines, and based on those lines, should return a pointer to a Polygon. Polygon* foo(Polygon& p){ std::vector<Line> lines = bar (p.begin(),p.end()); return new Polygon(lines); } Here's the question: I can always add a Polygon (vector Is there a better way that dereferencing each element of the vector and assigning it to the existing vector container? //for line in vector<Line*> v //vcopy is an instance of vector<Line> vcopy.push_back(*(v.at(i)) I think not, but I dont really like that approach. Hopefully, I will be able to convince the author of the class to change it, but I cant base my coding right now to that fact (and i'm scared of a performance hit). Thanks in advance.

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• C++ design question, container of instances and pointers

  - by Tom

  Hi all, Im wondering something. I have class Polygon, which composes a vector of Line (another class here) class Polygon { std::vector<Line> lines; public: const_iterator begin() const; const_iterator end() const; } On the other hand, I have a function, that calculates a vector of pointers to lines, and based on those lines, should return a pointer to a Polygon. Polygon* foo(Polygon& p){ std::vector<Line> lines = bar (p.begin(),p.end()); return new Polygon(lines); } Here's the question: I can always add a Polygon (vector Is there a better way that dereferencing each element of the vector and assigning it to the existing vector container? //for line in vector<Line*> v //vcopy is an instance of vector<Line> vcopy.push_back(*(v.at(i)) I think not, but I dont really like that approach. Hopefully, I will be able to convince the author of the class to change it, but I cant base my coding right now to that fact (and i'm scared of a performance hit). Thanks in advance.

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• C++ design question, container of instances and pointers

  - by Tom

  Hi all, Im wondering something. I have class Polygon, which composes a vector of Line (another class here) class Polygon { std::vector<Line> lines; public: const_iterator begin() const; const_iterator end() const; } On the other hand, I have a function, that calculates a vector of pointers to lines, and based on those lines, should return a pointer to a Polygon. Polygon* foo(Polygon& p){ std::vector<Line> lines = bar (p.begin(),p.end()); return new Polygon(lines); } Here's the question: I can always add a Polygon (vector Is there a better way that dereferencing each element of the vector and assigning it to the existing vector container? //for line in vector<Line*> v //vcopy is an instance of vector<Line> vcopy.push_back(*(v.at(i)) I think not, but I dont really like that approach. Hopefully, I will be able to convince the author of the class to change it, but I cant base my coding right now to that fact (and i'm scared of a performance hit). Thanks in advance.

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• C++ and Dependency Injection in unit testing

  - by lhumongous

  Suppose I have a C++ class like so: class A { public: A() { } void SetNewB( const B& _b ) { m_B = _b; } private: B m_B; } In order to unit test something like this, I would have to break A's dependency on B. Since class A holds onto an actual object and not a pointer, I would have to refactor this code to take a pointer. Additionally, I would need to create a parent interface class for B so I can pass in my own fake of B when I test SetNewB. In this case, doesn't unit testing with dependency injection further complicate the existing code? If I make B a pointer, I'm now introducing heap allocation, and some piece of code is now responsible for cleaning it up (unless I use ref counted pointers). Additionally, if B is a rather trivial class with only a couple of member variables and functions, why introduce a whole new interface for it instead of just testing with an instance of B? I suppose you could make the argument that it would be easier to refactor A by using an interface. But are there some cases where two classes might need to be tightly coupled?

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• Extensions methods and forward compatibilty of source code.

  - by TcKs

  Hi, I would like solve the problem (now hypothetical but propably real in future) of using extension methods and maginification of class interface in future development. Example: /* the code written in 17. March 2010 */ public class MySpecialList : IList<MySpecialClass> { // ... implementation } // ... somewhere elsewhere ... MySpecialList list = GetMySpecialList(); // returns list of special classes var reversedList = list.Reverse().ToList(); // .Reverse() is extension method /* now the "list" is unchanged and "reveresedList" has same items in reversed order */ /* --- in future the interface of MySpecialList will be changed because of reason XYZ*/ /* the code written in some future */ public class MySpecialList : IList<MySpecialClass> { // ... implementation public MySpecialList Reverse() { // reverse order of items in this collection return this; } } // ... somewhere elsewhere ... MySpecialList list = GetMySpecialList(); // returns list of special classes var reversedList = list.Reverse().ToList(); // .Reverse() was extension method but now is instance method and do something else ! /* now the "list" is reversed order of items and "reveresedList" has same items lake in "list" */ My question is: Is there some way how to prevent this case (I didn't find them)? If is now way how to prevent it, is there some way how to find possible issues like this? If is now way how to find possible issues, should I forbid usage of extension methods? Thanks.

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• Passing bundle to activity set as singletask

  - by Falmarri

  So I have a MapActivity that runs an asynchtask that occasionally updates what exactly it's displaying on the map (via a string). I originally pass this string in from the intent when the activity is first created. And then if you click on one of the drawables on the map, it opens a new activity, which can then create a new mapview (same class) with a different string setting. The problem I have is that I only want one instance of the mapview to be running at once. Thus I set android:launchmode="singletask" in the manifest. This works in that it brings the mapactivity to the front, but is there any way to send it a new intent bundle to get a new setting for the string it needs? I tried regetting the extras from the bundle, but it seems to retain the old bundle, not the new intent that was passed to it. I'm not sure I want to do startActivityForResult because the 2nd activity may or may not want to update the original activity. I hope that made sense. I can post code if necessary, but I think that should explain my situation.

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• How do I pass references as method parameters across AppDomains?

  - by Thiado de Arruda

  I have been trying to get the following code to work(everything is defined in the same assembly) : namespace SomeApp{ public class A : MarshalByRefObject { public byte[] GetSomeData() { // } } public class B : MarshalByRefObject { private A remoteObj; public void SetA(A remoteObj) { this.remoteObj = remoteObj; } } public class C { A someA = new A(); public void Init() { AppDomain domain = AppDomain.CreateDomain("ChildDomain"); string currentAssemblyPath = Assembly.GetExecutingAssembly().Location; B remoteB = domain.domain.CreateInstanceFromAndUnwrap(currentAssemblyPath,"SomeApp.B") as B; remoteB.SetA(someA); // this throws an ArgumentException "Object type cannot be converted to target type." } } } What I'm trying to do is pass a reference of an 'A' instance created in the first AppDomain to the child domain and have the child domain execute a method on the first domain. In some point on 'B' code I'm going to call 'remoteObj.GetSomeData()'. This has to be done because the 'byte[]' from 'GetSomeData' method must be 'calculated' on the first appdomain. What should I do to avoid the exception, or what can I do to achieve the same result?

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• Risky Business with LINQ to SQL and OR Designer?

  - by Toadmyster

  I have two tables with a one to many relationship in SQL 2008. The first table (BBD) PK | BBDataID | int       | Floor_Qty | tinyint       | Construct_Year | char(4)       | etc, etc describes the data common to all buildings and the second (BBDCerts) PK | BBDCertsID | int       | BBDataID | int       | Certification_Type | varchar(20)       | etc, etc is a collection of certifications for a particular building. Thus, the primary key in BBD (BBDataID) is mapped to the corresponding field in BBDCerts via an FK relationship, but BBDCertsID is the second table's primary key and BBDataID is not because it will not be unique. My problem is that I want to be able to use the OR generated data context to get at the list of certs when I access a particular record in the BBD table. For instance: Dim vals = (From q in db.BBD Where q.BBDataID = x Select q.Floor_Qty, q.Construct_Year, q.BBDCerts).SingleOrDefault and later be able to access a particular certification like this: vals.BBDCerts.Certification_Type.First Now, the automatic associations created when the SQL tables are dropped on the design surface don't generate the EntityRef associations that are needed to access the other table using the dot notation. So, I have to use the OR designer to make the BBDCerts BBDataID a primary key (this doesn't affect the actual database), and then manually change the association properties to the appropriate OneToMany settings. There might be a better way to approach this solution but my question is, is the way I've done it safe? I've done a barrage of tests and the correct cert is referenced or updated every time. Frankly, the whole thing makes me nervous.

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• JAXM soap message parsing

  - by Dean

  I am getting the following XML back from a .net service: <?xml version="1.0" encoding="utf-8"?> <soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema"> <soap:Body> <validateCredentialsResponse xmlns="http://www.paragon.com/positionmonitor/PositionMonitor"> <validateCredentialsResult> <ResultData xsi:type="ValidateCredentialsResultData"> <validated>true</validated> <alreadyLoggedIn>false</alreadyLoggedIn> </ResultData> <Status> <Condition xmlns="">SUCCESS</Condition> <ErrorCode xmlns="">BO.00000</ErrorCode> <ErrorDesc xmlns="">OK</ErrorDesc> </Status> </validateCredentialsResult> </validateCredentialsResponse> </soap:Body> </soap:Envelope> ...and I'm trying to parse it using JAXM, however the following always evaluates to null: SOAPEnvelope env = reply.getSOAPPart().getEnvelope(); Can anyone help me out here?

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• Is NFS capable of preserving order of operations?

  - by JustJeff

  I have a diskless host 'A', that has a directory NFS mounted on server 'B'. A process on A writes to two files F1 and F2 in that directory, and a process on B monitors these files for changes. Assume that B polls for changes faster than A is expected to make them. Process A seeks the head of the files, writes data, and flushes. Process B seeks the head of the files and does reads. Are there any guarantees about how the order of the changes performed by A will be detected at B? Specifically, if A alternately writes to one file, and then the other, is it reasonable to expect that B will notice alternating changes to F1 and F2? Or could B conceivably detect a series of changes on F1 and then a series on F2? I know there are a lot of assumptions embedded in the question. For instance, I am virtually certain that, even operating on just one file, if A performs 100 operations on the file, B may see a smaller number of changes that give the same result, due to NFS caching some of the actions on A before they are communicated to B. And of course there would be issues with concurrent file access even if NFS weren't involved and both the reading and the writing process were running on the same real file system. The reason I'm even putting the question up here is that it seems like most of the time, the setup described above does detect the changes at B in the same order they are made at A, but that occasionally some events come through in transposed order. So, is it worth trying to make this work? Is there some way to tune NFS to make it work, perhaps cache settings or something? Or is fine-grained behavior like this just too much expect from NFS?

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• MySQL Query Where Column Like Column

  - by shmeeps

  I'm working on a small project that involves grabbing a list of contacts which are stored for each group. Essentially, the database is set up so that each group has a primary and secondary contact stored as, unsurprisingly, Group.Primary and Group.Secondary. The objective is to pull every Primary and Secondary contact for each Group and display them in a sortable table. I have the sortable table all worked out, but I have come across a small problem. Each primary and secondary field can have more than one contact separated by a comma. For instance, if Primary contained 123,256 , it would need to pull both Contacts with IDs 123 and 256. I had intended to use a query formatted like this: SELECT * FROM Group G, Contacts C WHERE G.Primary LIKE %C.ID% OR G.Secondary LIKE %C.ID% so that I could just skip the comma part, but I can't seem to find a working query for this. My question to you is, am I just overlooking something here? Is there a simple query that would let me do this? Or am I better off getting the groups and contacts separately, and combine the two later. I think the former is a little easier to understand when read, which is a plus as this is a shared project, but if that is not possible I will do the latter. This code is simplified, but it gets the point across.

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• Parsing a Multi-Index Excel File in Pandas

  - by rhaskett

  I have a time series excel file with a tri-level column MultiIndex that I would like to successfully parse if possible. There are some results on how to do this for an index on stack overflow but not the columns and the parse function has a header that does not seem to take a list of rows. The ExcelFile looks like is like the following: Column A is all the time series dates starting on A4 Column B has top_level1 (B1) mid_level1 (B2) low_level1 (B3) data (B4-B100+) Column C has null (C1) null (C2) low_level2 (C3) data (C4-C100+) Column D has null (D1) mid_level2 (D2) low_level1 (D3) data (D4-D100+) Column E has null (E1) null (E2) low_level2 (E3) data (E4-E100+) ... So there are two low_level values many mid_level values and a few top_level values but the trick is the top and mid level values are null and are assumed to be the values to the left. So, for instance all the columns above would have top_level1 as the top multi-index value. My best idea so far is to use transpose, but the it fills Unnamed: # everywhere and doesn't seem to work. In Pandas 0.13 read_csv seems to have a header parameter that can take a list, but this doesn't seem to work with parse.

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• Generating all possible subsets of a given QuerySet in Django

  - by Glen

  This is just an example, but given the following model: class Foo(models.model): bar = models.IntegerField() def __str__(self): return str(self.bar) def __unicode__(self): return str(self.bar) And the following QuerySet object: foobar = Foo.objects.filter(bar__lt=20).distinct() (meaning, a set of unique Foo models with bar <= 20), how can I generate all possible subsets of foobar? Ideally, I'd like to further limit the subsets so that, for each subset x of foobar, the sum of all f.bar in x (where f is a model of type Foo) is between some maximum and minimum value. So, for example, given the following instance of foobar: >> print foobar [<Foo: 5>, <Foo: 10>, <Foo: 15>] And min=5, max=25, I'd like to build an object (preferably a QuerySet, but possibly a list) that looks like this: [[<Foo: 5>], [<Foo: 10>], [<Foo: 15>], [<Foo: 5>, <Foo: 10>], [<Foo: 5>, <Foo: 15>], [<Foo: 10>, <Foo: 15>]] I've experimented with itertools but it doesn't seem particularly well-suited to my needs. I think this could be accomplished with a complex QuerySet but I'm not sure how to start.

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• How to differentiate between method and function in a decorator?

  - by defnull

  I want to write a decorator that acts differently depending on whether it is applied to a function or to a method. def some_decorator(func): if the_magic_happens_here(func): # <---- Point of interest print 'Yay, found a method ^_^ (unbound jet)' else: print 'Meh, just an ordinary function :/' return func class MyClass(object): @some_decorator def method(self): pass @some_decorator def function(): pass I tried inspect.ismethod(), inspect.ismethoddescriptor() and inspect.isfunction() but no luck. The problem is that a method actually is neither a bound nor an unbound method but an ordinary function as long as it is accessed from within the class body. What I really want to do is to delay the actions of the decorator to the point the class is actually instantiated because I need the methods to be callable in their instance scope. For this, I want to mark methods with an attribute and later search for these attributes when the .__new__() method of MyClass is called. The classes for which this decorator should work are required to inherit from a class that is under my control. You can use that fact for your solution. In the case of a normal function the delay is not necessary and the decorator should take action immediately. That is why I wand to differentiate these two cases.

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• Is Storing Cookies in a Database Safe?

  - by viatropos

  If I use mechanize, I can, for instance, create a new google analytics profile for a website. I do this by programmatically filling out the login form and storing the cookies in the database. Then, for at least until the cookie expires, I can access my analytics admin panel without having to enter my username and password again. Assuming you can't create a new analytics profile any other way (with OpenAuth or any of that, I don't think it works for actually creating a new Google Analytics profile, the Analytics API is for viewing the data, but I need to create an new analytics profile), is storing the cookie in the database a bad thing? If I do store the cookie in the database, it makes it super easy to programatically login to Google Analytics without the user ever having to go to the browser (maybe the app has functionality that says "user, you can schedule a hook that creates a new anaytics profile for each new domain you create, just enter your credentials once and we'll keep you logged in and safe"). Otherwise I have to keep transferring around emails and passwords which seems worse. So is storing cookies in the database safe?

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• Saving Data to Relational Database (Entity Framework)

  - by sheefy

  I'm having a little bit of trouble saving data to a database. Basically, I have a main table that has associations to other tables (Example Below). Tbl_Listing ID UserID - Associated to ID in User Table CategoryID - Associated to ID in Category Table LevelID - Associated to ID in Level Table. Name Address Normally, it's easy for me to add data to the DB (using Entity Framework). However, I'm not sure how to add data to the fields with associations. The numerous ID fields just need to hold an int value that corresponds with the ID in the associated table. For example; when I try to access the column in the following manner I get a "Object reference not set to an instance of an object." error. Listing NewListing = new Listing(); NewListing.Tbl_User.ID = 1; NewListing.Tbl_Category.ID = 2; ... DBEntities.AddToListingSet(NewListing); DBEntities.SaveChanges(); I am using NewListing.Tbl_User.ID instead of NewListing.UserID because the UserID field is not available through intellisense. If I try and create an object for each related field I get a "The relationship between the two objects cannot be defined because they are attached to different ObjectContext objects." error. With this method, I am trying to add the object without the .ID shown above - example NewListing.User = UserObject. I know this should be simple as I just want to reference the ID from the associated table in the main Listing's table. Any help would be greatly appreciated. Thanks in advance, -S

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• Converting a soap message to a document using wssj

  - by ellander

  I have been supplied with a jar file that should enable me to connect to a 3rd party system. When I run the necessary class I get an error which states that the namespace for the prefix "SOAPENV" has not been declared. Using jad I have decompiled the class files to try suss out what is going on and the error seems to occur when a soap message is being converted to a document. Unfortunately I don't know much about wssj. Can anyone suggest what the problem might be? I'm wondering if it's an incompatibility issue as it's not a recently created jar file and the decompiled code looks different to all the examples I can find. For example, in this section the examples have addNamesSpaceDeclaration rather than addAttribute... MessageFactory factory = MessageFactory.newInstance(); SOAPFactory sf = SOAPFactory.newInstance(); SOAPMessage msg = factory.createMessage(); MimeHeaders headers = msg.getMimeHeaders(); headers.addHeader("Content-Type", "text/xml"); SOAPEnvelope env = msg.getSOAPPart().getEnvelope(); env.addAttribute(sf.createName("xmlns:xsi"), "http://www.w3.org/2001/XMLSchema-instance");

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• Call 32-bit or 64-bit program from bootloader

  - by user1002358

  There seems to be quite a lot of identical information on the Internet about writing the following 3 bootloaders: Infinite loop jmp $ Print a single character Print "Hello World". This is fantastic, and I've gone through these 3 variations with very little trouble. I'd like to write some 32- or 64-bit code in C and compile it, and call that code from the bootloader... basically a bootloader that, for example, sets the computer up to run some simple numerical simulation. I'll start by listing primes, for example, and then maybe some input/output from the user to maybe compute a Fourier transform. I don't know. I haven't found any information on how to do this, but I can already foresee some problems before I even begin. First of all, compiling a C program compiles it into one of several different files, depending on the target. For Windows, it's a PE file. For Linux, it's a .out file. These files are both quite different. In my instance, the target isn't Windows or Linux, it's just whatever I have written in the bootloader. Secondly, where would the actual code reside? The bootloader is exactly 512 bytes, but the program I write in C will certainly compile to something much larger. It will need to sit on my (virtual) hard disk, probably in some sort of file system (which I haven't even defined!) and I will need to load the information from this file into memory before I can even think about executing it. But from my understanding, all this is many, many orders of magnitude more complex than a 12-line "Hello World" bootloader. So my question is: How do I call a large 32- or 64-bit program (written in C/C++) from my 16-bit bootloader.

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• destructor being called by subclass

  - by zero

  I'm currently learning more about php objects and constructors/destructors, but i've noticed in my code that the parent class's destructor is being called twice, I thought it was because i was extending the first class to my second class and that the second class was calling it, but this is what the php docs say about that: Like constructors, parent destructors will not be called implicitly by the engine. In order to run a parent destructor, one would have to explicitly call parent::__destruct() in the destructor body. so if it is not being called by the subclass then is it because by extended the first class that i've made a reference to the parent class making it call itself twice or I'm I way off base here? the code: <?php class test{ public $test1 = "this is a test of a pulic property"; private $test2 = "this is a test of a private property"; protected $test3 = "this is a test of a protected property"; const hello = 900000; function __construct($h){ //echo 'this is the constructor test '.$h; } function x($x2){ echo ' this is fn x'.$x2; } function y(){ print "this is fn y"; } } $obj = new test("this is an \"arg\" sent to instance of test"); class hey extends test{ function hey(){ $this->x('<br>from the host with the most'); echo ' <br>from hey class'.$this->test3; } } $obj2 = new hey(); echo $obj2::hello; ?>

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• In Python, how to make data members visible to subclasses if not known when initializing an object ?

  - by LB

  The title is a bit long, but it should be pretty straightforward for someone well-aware of python. I'm a python newbie. So, maybe i'm doing things in the wrong way. Suppose I have a class TreeNode class TreeNode(Node): def __init__(self, name, id): Node.__init__(self, name, id) self.children = [] and a subclass with a weight: class WeightedNode(TreeNode): def __init__(self,name, id): TreeNode.__init__(self, name, id) self.weight = 0 So far, i think I'm ok. Now, I want to add an object variable called father in TreeNode so that WeightedNode has also this member. The problem is that I don't know when initializing the object who is going to be the father. I set the father afterwards with this method in TreeNode : def set_father(self, father_node): self.father = father_node The problem is then when i'm trying to access self.father in Weighted: print 'Name %s Father %s '%(self.name, self.father.name) I obtain: AttributeError: WeightedNode instance has no attribute 'father' I thought that I could make father visible by doing something in TreeNode.__init__ but i wasn't able to find what. How can i do that ? Thanks.

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• A Null Reference Exception

  - by Alex

  "Object reference not set to an instance of an object." using System; using System.Collections.Generic; using System.Linq; using System.Text; using Microsoft.Xna.Framework; using Microsoft.Xna.Framework.Content; using Microsoft.Xna.Framework.Graphics; namespace XNAdev { class Sprite { //The size of the Sprite public Rectangle Size; //Used to size the Sprite up or down from the original image public float Scale = 1.0f; //The current position of the Sprite public Vector2 Position = new Vector2(115, 0); //The texture object used when drawing the sprite private Texture2D mSpriteTexture; //Load the texture for the sprite using the Content Pipeline public void LoadContent(ContentManager theContentManager, string theAssetName) { mSpriteTexture = theContentManager.Load<Texture2D>(theAssetName); Size = new Rectangle(0, 0, (int)(mSpriteTexture.Width * Scale), (int)(mSpriteTexture.Height * Scale)); } //Draw the sprite to the screen public void Draw(SpriteBatch theSpriteBatch) { theSpriteBatch.Draw(mSpriteTexture, Position, new Rectangle(0, 0, mSpriteTexture.Width, mSpriteTexture.Height), Color.White, 0.0f, Vector2.Zero, Scale, SpriteEffects.None, 0); } } } I am very new at this C# so any help would be great. I have no idea what my error is.

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• Python "callable" attribute (pseudo-property)

  - by mgilson

  In python, I can alter the state of an instance by directly assigning to attributes, or by making method calls which alter the state of the attributes: foo.thing = 'baz' or: foo.thing('baz') Is there a nice way to create a class which would accept both of the above forms which scales to large numbers of attributes that behave this way? (Shortly, I'll show an example of an implementation that I don't particularly like.) If you're thinking that this is a stupid API, let me know, but perhaps a more concrete example is in order. Say I have a Document class. Document could have an attribute title. However, title may want to have some state as well (font,fontsize,justification,...), but the average user might be happy enough just setting the title to a string and being done with it ... One way to accomplish this would be to: class Title(object): def __init__(self,text,font='times',size=12): self.text = text self.font = font self.size = size def __call__(self,*text,**kwargs): if(text): self.text = text[0] for k,v in kwargs.items(): setattr(self,k,v) def __str__(self): return '<title font={font}, size={size}>{text}</title>'.format(text=self.text,size=self.size,font=self.font) class Document(object): _special_attr = set(['title']) def __setattr__(self,k,v): if k in self._special_attr and hasattr(self,k): getattr(self,k)(v) else: object.__setattr__(self,k,v) def __init__(self,text="",title=""): self.title = Title(title) self.text = text def __str__(self): return str(self.title)+'<body>'+self.text+'</body>' Now I can use this as follows: doc = Document() doc.title = "Hello World" print (str(doc)) doc.title("Goodbye World",font="Helvetica") print (str(doc)) This implementation seems a little messy though (with __special_attr). Maybe that's because this is a messed up API. I'm not sure. Is there a better way to do this? Or did I leave the beaten path a little too far on this one? I realize I could use @property for this as well, but that wouldn't scale well at all if I had more than just one attribute which is to behave this way -- I'd need to write a getter and setter for each, yuck.

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